Akg c3000b Low output all of a sudden

Did you have more detail to the question? Seems to be missing something at the end. Please Edit the question or Add Information or Comment to flesh it out
What in specific are you hooking the microphone into?
Amplifier, preamp, mixer, receiver?
It will depend what your input is expecting
Have you put in new batteries in the mic?
Do you see that the receiver is receiving the mic signal?
Is there any sound at all when you plug the output of the receiver into the input of your { fill in the blank } ?
Setup guide
https://www.akg.com/on/demandware.static/-/Sites-masterCatalog_Harman/default/dw3aa903a6/pdfs/AKG_WMS40_Mini2_Quickstart_Guide.pdf

Where are you getting the low output? It would be very strange if all channels failed in the same way so it is probably something common to them all. Can you get the clip light to illuminate at all by turning the gain up on a mic channel and shouting into the mic? You are referring to the gain at the top of the strip not the slider aren't you? What happens if you assign a channel to the other sub channel, does the level come up through that? What happens if you take the output from the aux send and you send signal there, is it the same issue?
I may be completely wrong but I suspect if it is only mics that are causing a problem you may have the routing set up wrong or not set a level somewhere else right.

Try building a little mike-preamp with an opamp to boost the signal.
Enough schematics on internet.
Or walk into an electronics shop for a ready to build package.
These are the things I can suggest; I don't know your audiobox.
Perhaps there is internally a pot-meter to adjust the gain of your mike; that would be the easiest solution.

If this is a (phantom) mic, it has to be plugged into a phantom plug on a mixer (round black thing with 3 metal posts in the middle. Darren Kelly-themasterofthrowdown@yahoo.com

you should be using these:

M-AUDIO - Fast Track Pro - 4 x 4 Mobile USB Audio/MIDI Interface

Have you tried using a another XLR cable..,or are the other mics phantom powered condensers..ie try to locate if its a mic problem or the cable..2 other points are 1 - XLR leads for phantom mics need to be XLR to XLR male to Female plugs[ends] 2- is that often an XLR to XLR cable will be not soldered or wired right,so that the power [phantom power is not getting to the mic] so ensure you have a proper cable for that job...Sorry i can't be more specific but from reading your problem all these issues come to mind..If you have ruled all these things out,then it could be the capsule or loose joint or component in the mic which means contact AKG,pro audio services or find a DIY mic person to help..G luck!!

Try the mike on another PA. Sometimes power phantom do weird things with electrical installations hum on the line. Let say commercial fridge.

How to repair a C3000B with low output.

I had the same problem, and was able to repair it myself. i've seen several other posts with the same description, and if you can use a soldering iron, you're golden. I just now repaired mine, and it took less than 10 minutes. One of the leads going from the PCB board to the capsule was broken loose at the solder joint. Mine has been dropped, so no surprise. Somehow, if you boost the signal enough, you still get sound, where I would have expected silence.

You need a star bit set (I have a little mult-mini screwdriver set that has them stored in the handle from home depot.) There are two silver screws in the bottom lip. Remove these and the whole screen housing slides off. Even better, the capsule assembly slides out too. There are two prongs that plug into a harness when inserted.

There are two black wires from said prongs to capsule leads. See if one is loose. If so, solder that ***** back on and its as good as new, and hot signal.

You also need small plyers, someting to strip the small. I know this is a late post, but maybe it will elp somone. Good luck.

constant unrelenting hum?
check the fuse housing and make sure it is setup for your local voltage(120/240)

I kept messing with it till I got the right combination. It works fine now.

Thank you,